Hereditary persistence of fetal hemoglobin (HPFH) is a non-fatal condition in which adults produce fetal hemoglobin throughout adulthood. Because sickle cell anemia is ameliorated by the presence of fetal hemoglobin, HPFH is believed to be an identifiable modifier locus for other blood disorders such as sickle cell anemia and thalassemia. Elucidation of this mechanism could lead to a treatment for sickle cell anemia and thalassemias. A series of families in Sardinia that display HPFH were genotyped with genetic markers from the putative regions for the trait, chromosomes 6 and X. After linkage analysis, flanking markers for both chromosomes were genotyped and subsequent statistical analyses were performed. Additional markers will be typed in an attempt to narrow the candidate regions.